THE HUMAN STERNUM 9 



Goette's observations*' differ, to some extent, from Rathke's, but 

 bear them out in the essential point under consideration. In Cnemidophorus 

 he found the sternum formed in two triangular halves, each portion being 

 united at its proximal end with the distal end of the corresponding coracoid, 

 and united at the lateral border with one rib. Later on he finds three ribs, 

 and still later four ribs, associated with the sternal border. At the same 

 time, he expresses his adherence to the view of the origin of the sternum 

 from the ribs. In Anguis fragilis he finds, as Rathke did, the sternum in 

 two lateral halves, and admits that at an early stage the union of the ribs 

 to the sternum cannot be demonstrated, or else they have already separated 

 from the sternum. He maintains, however, that the anlage of the sternum 

 is the distal end of the first rib, which is successively joined by the succeeding 

 ribs. 



{d) Birds. 



In the chick (PI. Ill, Figs. 27, 28) the formation of the sternum is 

 characterized by the presence, at an early stage, of two separate cartilaginous 

 halves, between which is suspended the heart in the pericardium, with a thin 

 connecting cellular layer beneath it, joining together the two halves of the 

 sternum. It is generally described as being derived on each side from the 

 ventral ends of costal elements (two cervical and five thoracic), of which 

 the two cervical lose connexion with the half sternum, and the fifth 

 thoracic ribs become abortive and disappear. 



I have made personal observations of the development of the sternum 

 in the chick, and may summarize my observations here. At the end of 

 the sixth day one finds no sternum and no ribs. The shoulder-girdle is 

 present in a cartilaginous form, but the clavicle is not distinct. During 

 the seventh day the halves of the sternum appear, cellular at first, and 

 later on becoming cartilaginous. The coracoids are joined to the cellular 

 sternum at first directly ; afterwards, when the sternum becomes cartila- 

 ginous, by the intervention of a joint cavity ; the ribs appear, become 

 cartilaginous, and grow forwards to get connected with the half sternum on 

 each side. The clavicles are now present and are becoming ossified. The 

 ninth day shows fusion of the halves of the sternum, and formation of the 

 keel out of the median border of each half sternum. 



